1. Field of the Invention
The present invention relates to a method and apparatus for removing volatile contaminants from liquids. More specifically, the present invention relates to removing volatile organic compounds (VOCs) such as methyl tertbutyl ether (MTBE) from water by transferring them to a supply of gas sparged through the water.
2. State of the Art
Many water sources and/or industrial waste waters contain VOCs introduced from such sources as chemical processing, petroleum production or industrial cleaning processes. Groundwater can also contain these substances due to pollution from underground tanks, surface chemical spills, migration of chemicals from waste disposal sites, etc. Environmental regulations place limitations on the amount of VOCs that may be present in water when released to the environment or when used for purposes such as irrigation or for human consumption. Accordingly, it is often necessary that a source of water must be treated to remove VOCs before it is suitable for its intended use.
The removal of VOCs from these waters has been accomplished by various methods and apparatus known in the art for many years. The stripping can be conducted by direct sparging of air through water columns or trays, packed towers, and a wide variety of other specifically designed equipment. Due to the chemical potential difference, VOCs contained in the water will transfer spontaneously into the air from air/water interface during the stripping, thus the concentration of the VOCs in the water is reduced.
One common approach for VOC removal of the “pump-and treat” category is the use of a packed tower stripper. In packed tower stripping, VOC-containing water is pumped and fed into the top of the tower and naturally flows to the bottom of the tower by passing through the packing media inside the tower. The packing media is used to increase the air/water contact as well as to increase the residence time of the water in the tower for stripping. The air is blown from the bottom of the tower and travels upward until discharged from the top of the tower. During such air/water countermotion, VOCs contained in the water are stripped from water into the air. Tower strippers of this type are shown, for example, in U.S. Pat. No. 4,608,163 to Yohe et al. and U.S. Pat. No. 5,378,267 to Bros et al.
Although widely used, the major disadvantage of the packed tower is the efficiency and the size. It basically has a low processing capacity (gallons treated per minute per cubic foot of the equipment volume). It is very common for commercial strippers to be 10 feet in diameter and 15-25 feet in height. As such, significant investment in terms of equipment and space is required. Further, dissolved metal ions such as Fe+2 and Ca+2 contained in the water, together with other solid particles in the water, often precipitate during the stripping, resulting in the fouling of the packing media. Frequent cleaning or replacement of the packing media is therefore needed, resulting in a high operational cost. In addition, from an engineering point of view it is very difficult and costly to scale-down the design of tower strippers to economically handle small flowrate applications. As an example, it is well know that many agricultural water wells in the state of California contain MTBE. Currently, there are no commercial packed tower strippers that are small enough, efficient enough, low cost enough and user friendly enough for farmers or ranchers to install them to remove MTBE under small flowrate situations. Further, MTBE has a high chemical affinity with the water. As such its volatile pressure is very low and stripping is very difficult. It has been reported in prior testing and experiments that the use of two packed tower strippers in series was required in order to achieve the desired stripping or removal of the MTBE. As such, equipment, operation costs and space considerations become significant.
Another approach to removing VOCs from water involves using a hydrocyclone type apparatus. In this method, a helical or swirl flow of contaminated water is generated within a cylindrical passageway and a gas is sparged through the flow to remove volatiles. U.S. Pat. No. 5,662,811, U.S. Pat. No. 5,531,904 and U.S. Pat. No. 5,529,701 to Grisham et al., for instance, disclose various apparatus embodiments of this type where a horizontal porous tube is contained within an outer jacket defining a gas plenum. Water is injected through the horizontal tube as a spiral flow along its inner surface. As the water passes through the tube, a gas is sparged through the spiral flow to strip out volatiles. After the water travels the length of the tube, it is passed into a liquid collection vessel for later use, while the volatile containing gas is separated out into one or more gas discharge assemblies.
While the hydrocyclone strippers disclosed in these patents have provided improvements over packed stripping towers, they still exhibit drawbacks in terms of operating efficiency and ability to remove volatiles. For example, because the porous tube is horizontally oriented, radial accelerations of up to 150 G are required to maintain the desired swirl flow of water along its interior. This requires high flow velocity and thereby larger pumping equipment. Furthermore, in attempting to remove volatiles in a single pass through the tube, the ratio of the sparging gas flowrate to that of the liquid has to be greater than 50 to 1. At this level the gas may disturb the spiral flow and pass through the water too rapidly to efficiently strip volatiles. What is needed is a low cost, high capacity, high efficiency, user friendly stripping apparatus that can be conveniently manufactured into different sizes to handle both voluminous streams for industrial application and small streams for agricultural or residential uses.